The conceptual design of high-speed underwater jet engine was conducted and demonstrated using high concentration of hydrogen peroxide as a green propellant. On the basis of the principles of steam jet ejector, the central water jet configuration was applied for the engine where a central water jet was surrounded by an annular steam flow. The annular steam flow as the primary fluid was the decomposition gas of hydrogen peroxide and cold water was the secondary fluid. The catalyst pack was composed of the alumina pellets doped with manganese oxides. The performance evaluation of the engine was performed without the flow field of the secondary fluid. The demonstrator successfully generated a practical propulsive force, which could be applied for the systems of unmanned underwater vehicles. For design of experiments, the nozzle exit areas were dealt with. It should be ensured that the exit area of nozzle was sufficiently large to prevent thrust instabilities due to the disturbance in the high-speed flows. In contrast, the excessively large area of the nozzle exit might decrease the efficiency of the engine performance in that the geometry of the engine failed to encourage the heat energy transfer between the primary and the secondary fluids.